1. Field of the Invention
This invention relates to flexible hot rolled steel sheets having a tensile strength of not more than 35 kgf/mm.sup.2 and a total elongation of not less than 50% and an improved deep drawability and good bending property and bulging property.
2. Related Art Statement
Lately, manufacturers for automobiles, domestic electrical articles and the like tend to use thin hot rolled steel sheets instead of cold rolled steel sheets from a viewpoint of the reduction of cost. However, conventional hot rolled steel sheet is poor in deep drawability as compared with cold rolled steel sheet, so that their application is restricted at the present.
The deep drawability of the steel sheet is dependent upon the elongation property of the starting material and r-value. As a cause which the conventional hot rolled steel sheet is hardly applied to the deep drawing, in the hot rolled steel sheet produced by the usual method, the level of r-value is not more than 1.0 and it is difficult to obtain the r-value of not less than 1.5 as in the cold rolled steel sheet.
A countermeasure for improving the drawback on the deep drawability in hot rolled steel sheets is roughly divided into two methods, one being a method of approaching the r-value to that of the cold rolled steel sheet as far as possible and the other being a method of largely improving the elongation property.
As to the first method, Japanese Patent laid open No. 55-97431, No. 60-77927 and the like have proposed a method wherein steel having the conventionally used chemical composition is used and hot rolled under such a finish hot rolling condition that the reduction is high or the strain rate is high or the reduction is carried out within a particular low temperature range to thereby improve the r-value.
However, even if the above method is adopted, the resulting r-value is 1.0-1.3 at most and does not reach to the level of the cold rolled steel sheet, so that the deep drawability is not yet satisfied at the present. Furthermore, if it is intended to obtain a higher r-value by the above method, it is necessary to largely change the hot rolling condition from the usually used range toward high reduction side and high speed rolling side, which finally exceeds over the range of rolling conditions selectable in the existing hot rolling mill.
On the other hand, as a proposal for the improvement of elongation property on the latter deep drawability, there are mentioned hot rolled steel sheets obtained by adding B to the chemical composition in the usual flexible hot rolled steel sheet and regulating the weight ratios of B/N and Mn/S within given ranges, respectively, as disclosed in Japanese Patent laid open No. 62-139849. However, the total elongation of such a hot rolled steel sheet is not more than 48%, and even when the best properties of the hot rolled steel sheet are compared with the properties of the cold rolled steel sheet, they are only equal to the level of the cold rolled steel sheet at most.